1. Field of the Invention
The present invention relates to image forming technology for forming an image by an electrophotographic method.
2. Description of the Related Art
In recent years, a tandem system is being increasingly employed to improve the image-forming speed of color image forming apparatuses that utilize an electrophotographic method. According to this system, the color image forming apparatus is provided with the same number of developers and photosensitive drums as the number of color materials, and forms a color image by sequentially transferring images of different colors onto a conveyance belt or printing medium. With a color image forming apparatus employing this tandem system, multiple factors are known to cause registration errors at the image forming units of each color. Various solutions have been proposed for the respective factors.
Such factors include unevenness between the lenses of a deflecting scanning device, displacement when attaching the image forming unit, and displacement when mounting the deflecting scanning device to the main body of the color image forming apparatus. Due to such factors, an inclination or skew arises in the scan lines. Since the extent of inclination or skewing differs for each color, registration errors occur. Japanese Patent Laid-Open No. 2004-170755 discloses a method for dealing with this kind of registration error. According to this method, the size of the inclination and skew of a scan line is measured using an optical sensor, bitmap image data is corrected so as to cancel out the inclination and skew, and an image is formed based on the corrected image data. This method does not require a mechanical adjustment member or an adjustment process during assembly, since registration errors are corrected by processing image data electrically. Consequently, registration errors can be dealt with at a low cost.
However, according to the method disclosed in Japanese Patent Laid-Open No. 2004-170755, a line buffer is required in order to perform blend processing with respect to the adjacent line above or below the current line at a skew portion of the scan line. The required capacity of the line buffer depends on the skew width of the scan line. For example, if the skew width of the scan line spans N lines of bitmap image data, a line buffer that can store N lines of image data is required. Since the value of N varies for each color image forming apparatus, the capacity of a line buffer mounted in each apparatus must correspond to a number of lines that exceeds the maximum value of such variance. The capacity of the line buffer consequently increases, and the circuitry size is also increased, leading to a rise in costs.
Further, although the method disclosed in Japanese Patent Laid-Open No. 2004-170755 implements skew correction for bitmap image data, no mention is made with respect to a case of dealing with tile image data. The term “tile image data” refers to data obtained by dividing an image represented by the image data into a plurality of tile shapes. In comparison to bitmap data for an entire image, simultaneous parallel processing and rotational processing can be performed more efficiently when using tile image data. The transfer efficiency of image data can also be improved by compressing tile image data. However, when handling tile image data, the conventional method that corrects skewing for bitmap image data unfortunately cannot be applied directly to tile image data. The present invention solves the aforementioned conventional problem.